1. Field of the Invention
The present invention relates to semipassive responders for communication systems and, in particular, to a "signpost" for vehicle location systems.
2. Description of the Prior Art
The aforementioned copending application Ser. No. 576,604 by Kaplan and Ritzie entitled "A Homodyne Communication System" describes a communication system, suitable for use in an automatic vehicle locating (AVL) system, wherein a homodyne interrogating unit transmits a continuous wave RF interrogation signal to illuminate a semipassive responder or signpost. The term "semipassive" as used herein refers to a responder which operates on a local source of d.c. energy but is not provided with a local source of R.F. energy, such R.F. energy being derived from the incident R.F. interrogation signal. The responder, in turn, radiates a coded response back to the interrogation unit. In one embodiment of such a homodyne communication system the responder provides a reply signal having impressed thereon a periodic differential phase shift of, for example, 180.degree. at one of two periodic rates, a mark or space frequency, in accordance with sequential bits of a coded message. The reply signal is subjected at the responder to a further periodic differential phase shift of, for example, approximately 90.degree., at a so-called "diversity rate." The sequence of mark or space frequency bits in the reply signal is detected in the homodyne interrogating unit to determine the coded message. The further phase shifting, hereinafter referred to as "phase diversity," is utilized to counteract nulls in the homodyne interrogating unit output signals caused by given relative phase relationships between the interrogation and reply signals. Thus, in the above-described embodiment the semipassive responder requires a phase modulator capable of generating differential phase shifts in excess of 180.degree., preferably in the vicinity of 270.degree., at the operating frequencies of the system. Such operating frequencies are typically in the 10 GHz range.
Phase modulators capable of producing differential phase shifts in substantial excess of 180.degree., and specifically in the vicinity of 270.degree., typically utilize a plurality of varactor diodes or a directional coupling device, such as one or more circulators or quadrature hybrid devices, or both. Such prior art varactor phase modulators are described in U.S. Pat. No. 3,479,615 issued Nov. 18, 1969 to R. V. Garver, entitled "Varactor Continuous Phase Modulator Having a Resistance in Parallel with the Varactor." Further description of such varactor phase modulators may be found in the following articles: R. N. Hardin et al, "Electronically-Variable Phase Shifters Using Variable Capacitance Diodes," Proceedings of the IRE, Vol. 48, pp. 944-945, May 1960; R. M. Searing, "Variable Capacitance Diodes Used as Phase-Shift Devices," Proceedings of the IRE (correspondence), Vol. 49, pp. 640-641, March 1961; C. S. Kim et al., "Varactor S-Band Direct Phase Modulator," IEEE Journal of Solid-State Circuits, Vol. SC-1, pp. 45-51, 1966; R. V. Garver, "360.degree. Varactor Linear Phase Modulator," IEEE Transactions on Microwave Theory and Technique, Vol. MTT-17, March 1969, pp. 137-147; and B. T. Henoch et al., "A 360.degree. Reflection-Type Diode Phase Modulator," IEEE Transactions on Microwave Theory and Techniques, January 1971, pp. 103-105.
Prior art responders utilizing varactor phase modulators, including a plurality of varactor diodes, a circulator or hybrid device, or both, have one or more limitations or constraints that render them unsuitable for environments with low energy sources. A phase modulator utilizing a plurality of varactor diodes consumes more power obviously than a phase modulator utilizing only a single varactor diode. Circulators and hybrid devices are relatively complex devices and represent an additional expense in the implementation of the responder even if, as implemented, only a single varactor is needed. For large scale usage of such responders, such as in an automatic vehicle location (AVL) system, it is desirable that the responder be simple and inexpensive and, to maximize battery life, consume as little power as practicable.